Currently, thin film transistor (hereinafter called TFT) with amorphous silicon thin film is utilized as a switching element of such as liquid crystal display devices which are put to practical use in such as monitors of television receiver or personal computers. However, field-effect mobility of the amorphous silicon is extremely small compared with crystalline silicon or polysilicon less than 100 cm2/Vs, it is approximately 0.5-1 cm2/Vs. Therefore, size of the TFT with amorphous silicon thin film, especially channel width must be large to ensure necessary amount of current to charge load capacitance with liquid crystal display getting larger and higher in resolution or higher speed response.
As the result, parasitic capacitance of the TFT increases, and the increased parasitic capacitance causes increase of auxiliary capacitance which has a function to decrease negative effects of the parasitic capacitance. The auxiliary capacitance is formed as a crossover capacitance of auxiliary capacitance line and pixel electrode, moreover the auxiliary capacitance line ought to be formed by opaque metal with low resistivity to decreases the time constant. Therefore, as the result, the increase of the auxiliary capacitance causes the increase of the area of the auxiliary capacitance line and also causes the decrease of aperture ratio. That is to say, there is a limit to keep on using TFTs with amorphous silicon thin film as the switching element of the pixel, because it comes difficulty to secure the enough aperture ratio with liquid crystal display getting larger and higher in resolution or higher speed response.
On the other hand, recently, semiconductor elements which use metal oxide semiconductor thin film attract attention. Although behavior of this thin film is different by film formation method or process condition, the field-effect mobility of 3 cm2/Vs or more in Soluble type or the field-effect mobility of 10 cm2/Vs or more in the film formation method by sputtering technique are indicated, and further higher field-effect mobility is expected based on the future study. Besides, this thin film has such as features of availability to form in lower temperature or transparency for visible light, and it is possible to form flexible and transparent TFT on transparent substrate such as plastic substrate or film (Patent document No. 1).
Besides, as oxide semiconductor film which is used in active layer of the TFT, semi-insulating and transparent amorphous thin film which is composed from the oxide including In, Ga and Zn is known. The structure of top gate type TFT which uses this in channel layer and uses the matter which laminates Au film on the layer of InGaZnO3(ZnO)4 which has the large electric conductivity as source-drain electrode is disclosed. Further, the prior art that the TFT of amorphous InGaZnO4 has the extremely large field-effect mobility compared with the amorphous silicon thin film transistor is disclosed (Patent document No. 2).
And then, active research and development for utilizing the TFT which has such the excellent characteristics is now performed in not only the liquid crystal display device but also the other display devices. Further, development of organic TFT which does not need expensive vacuum equipments as manufacturing equipments is performed actively, recently, the matter that the field-effect mobility exceeds 1 cm2/Vs is reported, and the research and development which purposes the application to the display device is performed energetically. Patent document No. 3 discloses the technology which decreases flicker which occurs by redistribution of the electrical charge and the time constant of the scanning line in Cs on gate type array substrate which forms the auxiliary capacitance between the pixel electrodes which are arranged by superimposing partly through the scanning line of previous stage of scanning direction and insulating film.